I agree that this is an internal refactoring for better subclassability and does not require new tests, only that all existing tests pass.

A case could be made that if we're refactoring widget implementations specifically to make them more easily subclassable, we should also document those class attributes intended for overriding; otherwise we're encouraging reliance on undocumented implementation details. I don't feel strongly enough about that to block this going in, though.